iio:core: mounting matrix support

Expose a rotation matrix to indicate userspace the chip placement with
respect to the overall hardware system. This is needed to adjust
coordinates sampled from a sensor chip when its position deviates from the
main hardware system.

Final coordinates computation is delegated to userspace since:
* computation may involve floating point arithmetics ;
* it allows an application to combine adjustments with arbitrary
  transformations.

This 3 dimentional space rotation matrix is expressed as 3x3 array of
strings to support floating point numbers. It may be retrieved from a
"[<dir>_][<type>_]mount_matrix" sysfs attribute file. It is declared into a
device / driver specific DTS property or platform data.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

1 files changed, 51 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index f155eff910f9..ba8df69d40b0 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -1512,3 +1512,54 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Raw (unscaled no offset etc.) pH reading of a substance as a negative
 		base-10 logarithm of hydrodium ions in a litre of water.
+
+What:           /sys/bus/iio/devices/iio:deviceX/mount_matrix
+What:           /sys/bus/iio/devices/iio:deviceX/in_mount_matrix
+What:           /sys/bus/iio/devices/iio:deviceX/out_mount_matrix
+KernelVersion:  4.6
+Contact:        linux-iio@vger.kernel.org
+Description:
+		Mounting matrix for IIO sensors. This is a rotation matrix which
+		informs userspace about sensor chip's placement relative to the
+		main hardware it is mounted on.
+		Main hardware placement is defined according to the local
+		reference frame related to the physical quantity the sensor
+		measures.
+		Given that the rotation matrix is defined in a board specific
+		way (platform data and / or device-tree), the main hardware
+		reference frame definition is left to the implementor's choice
+		(see below for a magnetometer example).
+		Applications should apply this rotation matrix to samples so
+		that when main hardware reference frame is aligned onto local
+		reference frame, then sensor chip reference frame is also
+		perfectly aligned with it.
+		Matrix is a 3x3 unitary matrix and typically looks like
+		[0, 1, 0; 1, 0, 0; 0, 0, -1]. Identity matrix
+		[1, 0, 0; 0, 1, 0; 0, 0, 1] means sensor chip and main hardware
+		are perfectly aligned with each other.
+
+		For example, a mounting matrix for a magnetometer sensor informs
+		userspace about sensor chip's ORIENTATION relative to the main
+		hardware.
+		More specifically, main hardware orientation is defined with
+		respect to the LOCAL EARTH GEOMAGNETIC REFERENCE FRAME where :
+		* Y is in the ground plane and positive towards magnetic North ;
+		* X is in the ground plane, perpendicular to the North axis and
+		  positive towards the East ;
+		* Z is perpendicular to the ground plane and positive upwards.
+
+		An implementor might consider that for a hand-held device, a
+		'natural' orientation would be 'front facing camera at the top'.
+		The main hardware reference frame could then be described as :
+		* Y is in the plane of the screen and is positive towards the
+		  top of the screen ;
+		* X is in the plane of the screen, perpendicular to Y axis, and
+		  positive towards the right hand side of the screen ;
+		* Z is perpendicular to the screen plane and positive out of the
+		  screen.
+		Another example for a quadrotor UAV might be :
+		* Y is in the plane of the propellers and positive towards the
+		  front-view camera;
+		* X is in the plane of the propellers, perpendicular to Y axis,
+		  and positive towards the starboard side of the UAV ;
+		* Z is perpendicular to propellers plane and positive upwards.